1. Field of the Invention
The present invention relates to a driving circuit of display devices such as TFT (Thin Film Transistor) liquid crystal display devices and the like and more particularly to the driving circuit used to display devices, which is capable of displaying multi-shades of gray.
2. Description of the Related Art
Development of a liquid crystal display device is recently prevailing which also stimulates further development of a driving circuit used for the liquid crystal device.
A driving circuit of the display device for 6-bit 240 output digital image data is described in Society for Information Display (SID) International Symposium digest of technical papers (S. Saito and K. Kitagawa of NEC Corp. Kanagawa, Japan, Vol. XXVI, pp. 257-260, Figure 1, 1995). FIG. 11 is a schematic block diagram showing the conventional driving circuit used for display devices described in the above literature.
The conventional driving circuit is provided with a 80-bit shift register circuit 51 into which a switching signal R/L and a clock signal CLK are inputted, both of which are adapted to switch an inputting/outputting direction of a start pulse signal SP. The start pulse SP is inputted into either of a terminal SPR or a terminal SPL in accordance with the switching signal R/L and is outputted from other terminal to an adjacent driving circuit. To this shift register 51 is connected to a data register circuit 52 in which data for 6-bit 3 outputs including D00 to D05, D10 to D15 and D20 to D25 is sequentially stored. To this data register circuit is connected a data latch circuit 53 into which a latch circuit STB is inputted. A gray shade voltage generating circuit 56 is provided, which is used to divided gray shade voltages including 9 voltage values from V0 to V8 and to output one gray shade voltage. Moreover, a gray shade voltage selecting circuit 54 is provided which is used to select one gray shade voltage out of 64 gray shade voltages outputted from the gray shade voltage generating circuit 56 based on image data transferred from the data latch circuit 53. The gray shade voltage selecting circuit 54 has 64 ROM decoders. Furthermore, an amplifier 55 in which an operational amplifier is built therein is also mounted, which is used to perform an impedance conversion of signals outputted from the gray shade voltage selecting circuit 54.
In the gray shade voltage generating circuit 56, the gray shade voltages of 9 values inputted from outside are divided to generate the gray shade voltages of 64 values. Such a voltage dividing method is generally called a “resistance string method”.
Moreover, the gray shade selecting circuit 54 is composed of, for example, an enhancement mode transistor and a depletion mode transistor.
In the conventional driving circuit having such configurations as described above, when the start pulse SP is inputted into the shift register circuit 51, data for 6-bit 3 outputs including D00 to D05, D10 to D15 and D20 to D25 is sequentially stored.
Next, when the latch signal STB is inputted into the data latch circuit 53, all digital image data that had been stored in the data register circuit 52 is transferred to the data latch circuit 53 and stored therein.
From the gray shade voltage generating circuit 56 is supplied gray shade voltages of 64 values to the gray shade voltage selecting circuit 54 and, when the digital image data is transferred to the data latch circuit 53, one gray shade voltage is selected out of the gray shade voltage of 64 values based on the digital image data and is outputted.
The voltage outputted from the gray shade voltage selecting circuit 54, after its impedance is converted by the operational amplifier embedded in the amplifier 55, is applied to liquid crystals implemented in the liquid crystal display device.
However, in the conventional driving circuit described above, though it is possible to generate 64 (6-bit) gray shade voltages without any problem, if gray shades exceeding 64 have to be generated, there are following various problems to be solved.
That is, according to a conventional resistance string method, as the number of shades of gray increases, a size of a chip for the gray shade selecting circuit 54 significantly increases. For example, in the case of a driver used for 65 shades of gray, the gray shade voltage selecting circuit must have 64 pieces of ROM decoders per one output while, in the case of a driver used for 256 shades of gray, the gray shade voltage selecting circuit must have 256 pieces of ROM decoders (i.e., 4 times larger than 64 ROM decoders) per one output. Therefore, if these drivers must be implemented on a semiconductor integrated circuit, a device area must be 4 times as large as that for 64 shades of gray, thus causing a significant increase in the size of the chip.
Also, in the case of the driving circuit used for 64 shades of gray, since the gray shade voltage selecting circuit 54 has 64 ROM decoders, checking of operations of all these 64 decoders is required accordingly. In the case of the driving circuit used for 256 shades of gray, it is also necessary to check operations of all 256 decoders as well. Because of this, time required for testing is increased by four times, thus increasing test time required in an inspection process in a semiconductor circuit production and resulting in increased costs for testing.